Consider a matrix X ∈ R n × m One compact yet unclear notation to...

Graham Beasley

Graham Beasley

Answered

2022-07-16

Consider a matrix X R n × m
One compact yet unclear notation to select a row or column from this matrix is:
x X
How do you clearly select a row or column from a matrix?
I know X = ( x i j ) is a standard notation to select elements. Though I haven't seen this used, x i X i j for rows and x j X i j for columns might make sense. This is motivated by a similar notation I have seen, namely i X i j for row sum or j X i j for column sum.

Answer & Explanation

dasse9

dasse9

Expert

2022-07-17Added 12 answers

You could for example use elementary vectors. Let e j be an m × 1 vector of zeros with a one in j-th position. Then
x j = X e j
would be the j-th column of X.

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